1. Field of the Invention
This invention relates a polymer composition comprising as main components an ethylene-vinyl alcohol base copolymer and a specific block copolymer and also to usage of said polymer composition.
2. Related Art of the Invention
Ethylene-vinyl alcohol base copolymers have come to be used for various applications such as food packaging materials because they have high-grade barrier properties against many gases and organic liquids and different from a polyvinylidene chloride resin or polyvinyl chloride resin, they do not emit harmful gases at the time of disposal by incineration. For example, in addition to the application to food packaging materials making use of high-grade gas-barrier properties, that to automobile gasoline tanks making use of high-grade barrier properties against an organic liquid are actualized or under investigation.
Ethylene-vinyl alcohol base copolymers, however, are inferior in flexibility so that they are generally used as a composition or laminate with a flexible resin such as a polyolefin.
Ethylene-vinyl alcohol base copolymers generally have low affinity and inferior compatibility with other resins so that a composition obtained by mixing an ethylene-vinyl alcohol base copolymer with a flexible resin tends to substantially lose the barrier properties which the copolymer originally has. The laminate formed of a layer of a flexible resin and a layer of the ethylene-vinyl alcohol base copolymer, on the other hand, has improved flexibility compared with the single layer of the ethylene-vinyl alcohol base copolymer but is insufficient in flexibility in some applications.
A tire tube for bicycles, a container packing for hermetic sealing between a beverage bottle and a crown cap therefor or hermetic sealing between a pharmaceutical bottle and a lid therefor or the like is requested to have both gas-barrier properties and flexibility or rubber elasticity. For example, IIR (butyl rubber) is generally used for the tire tube for bicycles to satisfy the high-grade gas-barrier properties and rubber elasticity necessary for the tire tube, while NR (natural rubber) or IIR is generally used for the container packing to satisfy the flexibility, rubber elasticity and gas-barrier properties necessary for the packing.
In order to allow molded or formed products, which have been obtained using the above-described NR or IIR, to manifest rubber elasticity, however, a cumbersome vulcanization step is added subsequent to a molding or forming step. Since NR has not so high gas-barrier properties, a container packing or the like obtained using it is disadvantageous in the long-term shelf stability of the contents enclosed in the container.
Proposed as a polymeric material having excellent flexibility and rubber elasticity and besides not requiring a vulcanization step are thermoplastic elastomers such as SEBS polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer! and SEPS polystyrene-poly(ethylene-propylene)-polystyrene triblock copolymer!, but their gas-barrier properties are not always sufficient.
As another thermoplastic elastomer, a block copolymer having a polymer block of an aromatic vinyl monomer and a polymer block of isobutylene is known and its applications to a pharmaceutical rubber stopper, a pharmaceutical container and the like are proposed (Japanese Patent Application Laid-Open No. 5-212104 and Japanese Patent Application Laid-Open No. 5-269201). The gas-barrier properties of the above elastomer are however insufficient in some applications.
In European Patent Application Laid-Open No. 572,667, described is a process for improving impact resistance of a thermoplastic resin, which comprises incorporating 0.5 to 35 parts by weight of the above-described block copolymer having a polymer block of an aromatic vinyl monomer and a polymer block of isobutylene in 100 parts by weight of the thermoplastic resin. Examples of the thermoplastic resin include polycarbonate, polyphenylene ether, polystyrene, polyolefin, polyester, polyvinyl chloride, styrene-methacrylate-acrylonitrile copolymer, methacrylate-styrene copolymer, acrylonitrile-styrene copolymer, polymethyl methacrylate, polyphenylene sulfide and polyvinyl acetate. No description of an ethylene-vinyl alcohol base copolymer is included in it. In the above literature, it is described that the above resin composition composed of a thermoplastic resin and a block copolymer has good heat resistance, impact resistance, solvent resistance and compatibility but a description of barrier properties against gases or the like is not included.